DC-DC converters are devices which are employed to convert an input DC voltage to another DC voltage. DC-DC converters may be classified generally as linear or switching. A conventional arrangement for a switching DC-DC converter uses a power stage comprising one or more switching devices and one or more inductors or capacitors or both to convert an input voltage (Vin) to an output voltage (Vout). A controller is employed to try and maintain the output voltage at a desired set point. Conventionally, pulse width modulation is employed to control the operation of the switching devices within the power stage and accordingly the controller provides a control signal to a PWM module which operates the switching devices. A variety of different switching circuit topologies may be employed within the power stage which will be familiar to those skilled in the art, including for example the conventional buck and boost topologies. In a linear switching supply, the gain of a transistor or similar device is adjusted to achieve a desired output from an input voltage.
The output from the DC-DC converter is typically provided to a load which may be an electronic circuit comprising a plurality of components or just a single component, for example an LED light.
To improve the performance of the controller it is known to use adaptive control, an example of which is for example described in U.S. Ser. No. 12/439,802, which is assigned to the present assignee, the entire contents of which are hereby incorporated by reference. The use of adaptive control significantly improves the performance of controllers. In the referenced patent application, two controllers are provided which operate jointly in parallel and whose outputs are combined together in a balanced manner. The adaptive control function alters the balance point between the two controllers to bias the control towards one or other of the controllers. An advantage of this approach is that stability is ensured notwithstanding the control function being changed by the adaptive elements. In these arrangements, the adaptive controller is effectively tuned with a single degree of freedom, which provides stability in the tuning process by allowing the overall control function to vary between a cautious and an aggressive function.
Adaptive controllers require excitation in order provide the necessary information for adaptation to occur. Where persistent excitation does not exist, the adaptive controller may not tune optimally, or may fail to tune at all. It is possible to excite the adaptive controller by introducing an external stimulus such as pseudo-random noise, or a periodic signal, but the disturbance this causes to voltage regulation is often unacceptable. The term external used in this context refers to the excitation being external to the adaptive control circuit, rather than its physical characteristics. Similarly a non-linearity may be introduced into the control loop to introduce a disturbance. Thus for example, in U.S. Pat. No. 7,586,767 which measures the characteristics of limit cycle oscillations to perform the tuning function, the limit cycle oscillations are caused by introducing a non-linearity in the control loop by altering the resolution of the PWM circuit. However again a disturbance is being introduced that may affect voltage regulation.
The present application seeks to improve the operation of adaptive controllers as employed generally in power supplies.